As pointed out in Kelly et al. application Ser. No. 508,274, the operation of a dredge crane operating a grab may be considered having plural sides, such as a two-sided clamshell bucket in underwater dredging. Such involves the sequential steps of lowering, closing, raising, luffing out, opening, luffing in, and again lowering, etc. under changing bottom depths and flowing water. Objectionably, slack hoist lines from poor timing of the braking operation can permit the "burying" of a bucket and incur many kinds of delaying complications including the tilting or tumbling of the bucket, improper closing of the bucket, and, undue stress movements on the hoist.
Accordingly, of the three steps of lowering, closing and raising the bucket, the lowering which includes the braking of the winch is the most critical regarding uniform loading of the bucket and time saving performance, and, all these are the most difficult to program either manually or otherwise, because they occur out of sight of the crane operator. Even if visible through clear water over a range of depths it would be difficult to determine visually when the bucket has established a desirable working position with respect to the bottom and is ready for closing to pick up a load in a single pass without either "burying" the bucket or "skimping" the load.
Full automation for a wide range of changing depths and conditions is highly desirable for these three steps. However, confusing conditions are experienced because in the above sequence two non-contiguous steps occur when the bucket is the lightest on its hoist winch drive, namely, when the winch brake is applied for luffing and opening the bucket, and when the bucket engages the bottom ready for closing. Also, two non-contiguous steps occur when the bucket is the heaviest on the winch drive, namely, when being raised and when being lowered. In both, similarities between the alternative steps provide like conditions that are substantially indistinguishable for automated partial automation. Additionally, two power efforts are involved and their coordination and control have to be transmitted to the bucket through separate connections additionally carried by the boom. One effort is to open and close the bucket, and the other effort is to raise and lower the bucket. Heretofore, these efforts have generally been controlled separately and manually with a substantial waste of operational time and power. Accordingly, time and labor saving operative automation has heretofore been limited.
In the Kelley et al application mentioned, electrical controls are used activated by pressure changes in the high pressure hydraulic connection between the pump and winch motor as related to pressure changes which can occur in either direction of movement.